In a typical cellular radio system, wireless devices, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The wireless devices can be mobile stations or user equipments (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability (e.g., mobile termination), and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data via radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The radio base stations communicate over the air interface operating on radio frequencies with the wireless device within range of the radio base stations.
In some versions, particularly earlier versions, of the radio access network, several radio base stations are typically connected, e.g., by landlines or microwave, to a radio network controller (RNC). The RNC, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural radio base stations connected thereto. The RNCs are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for wireless devices. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base stations are connected directly to a core network rather than to RNCs. In general, in LTE the functions of an RNC are performed by the radio base stations. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
The following description, for purposes of explanation, refers to LTE, WCDMA, UTRAN or E-UTRAN. This does however not limit the applicability to other technologies.
Dual Connectivity for e.g. LTE
With the proliferation of user friendly smart phones and tablets, the usage of high data rate services such as video streaming over the radio communications network is becoming commonplace, greatly increasing the amount of traffic in radio communications networks. Thus, there is a great urgency in the radio communications network community to ensure that the capacity of radio communications networks keeps up increasing with this ever-increasing user demand. The latest systems such as the LTE, especially when coupled with interference mitigation techniques, have spectral efficiencies very close to a theoretical Shannon limit. The continuous upgrading of current radio communications networks to support the latest technologies and densifying the number of radio base stations per unit area are two of the most widely used approaches to meet the increasing traffic demands.
Yet another approach that is gaining high attention is to use Heterogeneous Networks where the traditional pre-planned macro base stations or macro radio base stations, also known as the macro layer, are complemented with several low-powered base stations or low powered radio base station, such as pico nodes, that may be deployed in a relatively unplanned manner. The 3rd Generation Partnership Project (3GPP) has incorporated the concept of Heterogeneous Networks as one of the core items of study in the latest enhancements of LTE.
In a dual connectivity the idea is that the wireless device keeps connectivity to more than one radio base station at the time. Assuming that the macro radio base stations are able to provide coverage and the pico radio base stations are there only for capacity enhancements, i.e. no coverage holes, one alternative architecture is where the wireless device maintains a macro connectivity all the time, connected to a first radio base station being a Master eNB (MeNB)), and adds a pico connectivity when it is in the coverage area of the second radio base station also referred to as being connected to a Secondary eNB (SeNB)). When both connections are active or set up, the wireless device may get data and/or control signaling from many radio base stations. Dual connectivity to a wireless device from two radio base stations in a radio communications network may in some cases not schedule transmissions in an efficient manner leading to a non optimal use of dual connectivity.